In recent years, particulates, contained in exhaust gases discharged from internal combustion engines of vehicles such as buses, trucks, construction machines and the like, have raised serious problems as contaminants harmful to the environment and the human body.
For this reason, there have been proposed various ceramics filters which allow exhaust gases to pass through porous ceramics to collect particulates in the exhaust gases, so that the exhaust gases can be purified.
Conventionally, with respect to the ceramics filters of this type, as shown in FIG. 9, there has been known a column-shaped honeycomb structural body 140 in which a number of through holes 141 are placed in parallel with one another in the length direction with a partition wall 143 interposed there between.
As shown in FIG. 9(b), the through hole 141 is sealed with a sealing material 142 at one of its exhaust-gas inlet side and outlet side, so that exhaust gases that have entered one through hole 141 are discharged from another through hole 141 after having always passed through a partition wall 143 that separates the through holes 141.
In other words, when the honeycomb structural body 140 is placed in an exhaust gas passage of an internal combustion engine, particulates in exhaust gases discharged from the internal combustion engine are captured by the partition wall 143 when passing through the honeycomb structural body 140, so that the exhaust gases are purified.
Moreover, with respect to such an exhaust gas filter, a filter having the following structure has been proposed: through holes of two kinds, that is, a through hole with the end on the exhaust-gas outlet side being sealed (hereinafter, referred to as inlet-side through hole) is prepared as a through hole with a larger capacity (hereinafter, also referred to as large-capacity through hole) and a through hole with the end on the exhaust-gas inlet side being sealed (hereinafter, referred to as outlet-side through hole) is prepared as a through hole with a smaller capacity (hereinafter, also referred to as small-capacity through hole), so that the aperture ratio on the exhaust-gas inlet side is made relatively greater than the aperture ratio on the exhaust-gas outlet side.
FIG. 10 schematically shows a cross section perpendicular to the length direction of an exhaust gas filter disclosed in U.S. Pat. No. 4,417,908 (corresponding to JP Kokai Sho 58-196820, JP Kokoku Hei 3-49608 (hereinafter, referred to as Patent Literature 1)).
This exhaust-gas filter 60 has a cross-sectional structure in which squares, each smaller than each of square shapes constituting the checker board pattern, are placed on intersections of the checker board pattern, and this structure is constituted by small-capacity through holes 61b corresponding to the smaller squares and large-capacity through holes 61a located on the periphery thereof, with the partition wall 62a, 62b being formed between these through holes.
Moreover, FIGS. 11(a) to 11(d) schematically show cross-sections perpendicular to the length direction of exhaust-gas filters disclosed in U.S. Pat. No. 4,364,761 (corresponding to JP Kokai Sho 56-124417 and JP Kokai Sho 62-96717 (hereinafter, referred to as Patent Literature 2) and U.S. Pat. No. 4,276,071 (corresponding to JP Kokai Sho 56-124418)).
In these exhaust-gas filters 300 to 330, large-capacity through holes 301a, 311a, 321a, 331a and small-capacity through holes 301b, 311b, 321b, 331b having various shapes are formed, and partition walls 302, 312, 322, 332 are formed between these through holes.
Each of the partition walls 302, 312, 322, 332 separates each of the large-capacity through holes 301a, 311a, 321a, 331a and each of the small-capacity through holes 301b, 311b, 321b, 331b from one another, and there are substantially no partition wall that separates the large-capacity through holes 301a, 311a, 321a, 331a from each other.
In general, these filters have an increased pressure loss as particulates in exhaust gases are collected. Accordingly, the back pressure increases and, when the back pressure has exceeded a predetermined value, the load imposed on an engine or the like becomes greater, so that a recovery process needs to be carried out to eliminate the particulates. Therefore, the degree of pressure loss with elapsed time forms an important factor in evaluating the filter performances.
FIG. 1 is a conceptual diagram that shows main factors giving influences to the pressure loss.
As shown in FIG. 1, the main factors that give influences to the pressure loss include {circle around (1)} aperture ratio on the filter inlet side: ΔPa, {circle around (2)} friction upon passage through holes (inlet side {circle around (2)}-1: ΔPb-1, outlet side {circle around (2)}-2: ΔPb-2), {circle around (3)} resistance upon passage through a partition wall: ΔPc and the like.
Moreover, FIG. 2 is a graph that schematically shows variations in the pressure loss with elapsed time in each of the various exhaust-gas filters.
In the case of exhaust-gas filters having two types of through holes disclosed in Patent Literatures 1, 2 and the like, in comparison with the exhaust-gas filter in which the cross-sectional shape is a square shape, as shown in FIG. 9, with all the through holes having almost the same capacity, in a state prior to collection of particulates, a pressure loss derived from the aperture ratio on the filter inlet side and friction exerted upon passage through inlet-side through holes ({circle around (1)}: ΔPa+{circle around (2)}-1: ΔPb-1) is slightly reduced; however, a pressure loss derived from friction exerted upon passage through outlet-side through hole and resistance exerted upon passage through a partition wall ({circle around (2)}-2: ΔPb-2+{circle around (3)}: ΔPc) is increased. Consequently, it has been confirmed that the pressure loss prior to collection of particulates becomes higher in comparison with the exhaust-gas filter in which all the through holes have substantially the same capacity as shown in FIG. 9.
Further, in the case of an exhaust-gas filter in which most of wall portion of an exhaust-gas filter are constituted by wall portion (i) which is shared by one large-capacity through hole and the adjacent large-capacity through hole in the cross section perpendicular to the length direction and wall portion (ii) which is shared by one large-capacity through hole and the adjacent small-capacity through hole in the cross section perpendicular to the length direction, the pressure loss is varied depending on the ratio of these two kinds of wall portion.
For example, supposing that the aperture ratio is constant, when the rate of the wall portion (i) is great, it becomes difficult for exhaust gases to directly pass through the wall portion (ii) from the large-capacity through hole to flow into the small-capacity through hole; therefore, the pressure loss prior to collecting particulates (T0) tends to become higher.
However, since particulates are accumulated on the surface of the wall portion (ii) as the particulates are collected, the flow of exhaust gases that once enters the wall portion (i) and are transmitted over the porous wall to flow into the wall portion (ii) is under less resistance than the flow of exhaust gases that directly pass through the wall portion (ii) from the large-capacity through hole to flow into the small-capacity through hole, with the result that particulates are accumulated evenly over the entire wall portion constituting the large-capacity through hole. Therefore, the thickness of the particulates to be accumulated over the wall portion is reduced, so that it becomes possible to reduce the rising rate (ΔP3/(T1−T0)) of the pressure loss that increases as particulates accumulate.
Here, in the case where, in contrast, this rate is small, although the pressure loss prior to collecting particulates (T0) becomes lower, the rising rate (ΔP3/(T1−T0)) of the pressure loss that increases as particulates accumulate tends to increase.
In the exhaust-gas filter 60 disclosed in Patent Literature 1 (FIG. 10), the rate of wall portion (i) which is shared by the adjacent large-capacity through holes is comparatively great. For this reason, as shown in FIG. 2, the pressure loss (hereinafter, referred to as initial pressure loss) prior to collecting particulates (T0) becomes higher due to high resistance ({circle around (3)}: ΔPc) upon passage through a partition wall, and the pressure loss upon collecting particulates (T1) also becomes higher since the initial pressure loss is too high.
Therefore, from the viewpoint of engine management, a recovery process needs to be carried out before a prescribed amount of particulates has been accumulated. In other words, since the initial pressure loss is too high, the exhaust-gas filter 60 has a problem that substantially only a limited amount of particulates is collected.
Moreover, in the case of exhaust-gas filters 300 to 330 disclosed in Patent Literature 2 (FIG. 11), partition walls (i) separating the large-capacity through holes 301a, 311a, 321a and 331a from each other are in a point-contact state, and hardly exist.
For this reason, as shown-in FIG. 2, due to a high rising rate (ΔP3/(T1−T0)) of the pressure loss that increases as particulates accumulate, the pressure loss upon collecting particulates (T1) becomes too high.
Therefore, from the viewpoint of engine management in the same manner, a recovery process needs to be carried out before a prescribed amount of particulates have been accumulated. In other words, since the rising rate of pressure loss upon collecting particulates is high, the exhaust gas filters 300 to 330 have a problem that substantially only a limited amount of particulates is collected.
With respect to another conventional technique, microfilms of Japanese Utility Model Application No. 56-187890 (J UM Kokai Sho 58-92409 (see FIG. 6, page 4), hereinafter, referred to as Patent Literature 3), have disclosed a honeycomb structural body with cell pitches of large-capacity through holes being set almost in a range from 1.0 to 2.5 mm.
JP Kokai Hei 5-68828 (Japanese Patent gazette No. 3130587 (page 1), hereinafter, referred to as Patent Literature 4) has disclosed a honeycomb structural body in which the capacity rate of the large-capacity through holes is set to 60 to 70% while the capacity rate of the small-capacity through holes is set to 20 to 30%, with the cell pitch of the large-capacity through holes being set to almost in a range from 2.5 to 5.0 mm.
FIG. 19 is a cross-sectional view that schematically shows a cross section perpendicular to the length direction (hereinafter, simply referred to as cross section) of this honeycomb structural body 200, and this honeycomb structural body 200 has a structure in which small-capacity through holes 202, each having a triangular shape in its cross section, are placed on the periphery of a large-capacity through hole 201 having a hexagonal shape in its cross section.
Moreover, JP Kokai 2001-334114 (see FIG. 2, page 5) (WO 02/100514, hereinafter, referred to as Patent Literature 5) has disclosed a honeycomb structural body in which the ratio of the total area of the cross-section of small-capacity through holes to the total area of the cross-section of large-capacity through holes is set in a range from 40 to 120%.
FIG. 20 is a cross-sectional view that schematically shows a cross section perpendicular to the length direction of such a honeycomb structural body, and in this honeycomb structural body 210, small-capacity through holes 212, each having an laterally elongated hexagonal shape in its cross section, are placed on the periphery of a large-capacity through hole 211 having a right hexagonal shape in its cross section. Moreover, in the vicinity of the circumference thereof, the large-capacity through holes 211 having a right hexagonal shape and large-capacity through holes 213 having a trapezoidal shape are placed also in parallel with each other.
Furthermore, another structure in which the number of inlet-side through holes is made greater than the number of outlet-side through holes, so that the aperture ratio on the exhaust-gas inlet side is relatively greater than the aperture ratio on the exhaust-gas outlet side has also been disclosed (for example, see FIG. 3 of Patent Literature 1).
In the honeycomb filter of this type also, in comparison with the exhaust-gas filter in which the number of the cells on the inlet side and the number of the cells on the outlet side are different from each other and the cross-sectional shape shown in FIG. 9 is a quadrangle, with all the through holes having almost the same capacity, in a state prior to collection of particulates, a pressure loss derived from the aperture ratio on the inlet side and friction exerted upon passage through inlet-side through holes ({circle around (1)}: ΔPa-2+{circle around (2)}-1: ΔPb-1) is slightly reduced; however, the pressure loss derived from friction exerted upon passage through outlet-side through holes and resistance exerted upon passage through a partition wall ({circle around (2)}-2: ΔPb-2+{circle around (3)}: ΔPc) is increased. Consequently, the pressure loss prior to collection of particulates becomes higher in comparison with the exhaust-gas filter in which all the through holes have substantially the same capacity as shown in FIG. 9.
Moreover, U.S. Pat. No. 4,416,676 and U.S. Pat. No. 4,420,316 (hereinafter, referred to as Patent Literature 6) have disclosed techniques for adjusting the thickness of walls and physical properties; however, it was difficult to reduce the pressure loss by using only these techniques.
Further, JP Kokai Sho 58-150015 has disclosed a filter which is provided with square through holes and rectangular through holes; however, since this filter has a reformed cross section, it is difficult to manufacture the filter through an extrusion-molding process, and it is also difficult to mass produce the filter. Also, even without the reform of the cross section, the resistance of gases on the outlet side becomes higher to cause high pressure loss, since there is a difference between those having relatively large cross section and those having relatively small cross section in through holes on the outlet side.
In order to solve this conventional problem, filters having two types of through hole shapes, that is, in particular, a filter constituted by an octagonal shape and a square shape has been proposed (see the specification of French Patent No. 2789327, WO 02/10562).
It has been confirmed that, by forming the filter into such shapes, the pressure loss is improved. However, the results of various experiments carried out on various shapes and aperture ratios has shown that it is difficult to satisfy both of low pressure loss and high crack limit, and consequently, the amount of collection of particulates is limited. In addition, these filters are inferior in isostatic strength and compression strength.
Moreover, with respect to a conventional technique, WO 03/20407 has disclosed a honeycomb structural body in which two types of through holes, that is, relatively large square-shaped through holes and small through holes, are provided.
However, this filter still has high pressure loss, and is poor in isostatic strength and compression strength, thus, it was difficult to increase crack limit thereof.
As described above, in all the honeycomb structural bodies described in conventional techniques, since the aperture ratio on the exhaust-gas inlet side is made relatively greater in comparison with the honeycomb structural body in which the aperture ratio on the exhaust-gas inlet side and the aperture ratio on the exhaust-gas outlet side are equal to each other, it becomes possible to increase the limit of particulate collection, and also to lengthen the period up to the recovery process, when used as an exhaust gas purifying filter.
However, it has been found that, in comparison with a normal filter (that is, a filter in which the aperture ratios on the exhaust-gas inlet side and outlet side are the same), these filters have considerably high initial pressure loss. In addition, these filters cause degradation in strength. Therefore, these fail to satisfy properties such as high isostatic strength, high compression strength and high crack limit, as well as maintaining a low pressure loss.